In direct imaging electron microscopy, which is unrelated to the scanning method, one distinguishes basically between transmission electron microscopes and electron emission microscopes.
In transmission electron microscopes, the sequence of the modules is analogous to that of the light microscope, where the filament lamp is replaced by the electron source and electromagnetic lenses are used for the imaging system. The imaging takes place on a phosphor screen with a photo device. In a transmission electron microscope, pictures can be obtained by means of electron beam diffraction by simply switching. To this end, the excitation of the intermediate lens, i.e. with respect to the passage of current, is attenuated to such an extent that it can no longer reproduce the one-step enlarged image of the objective lens, but rather the preceding diffraction pattern of the specimen, which is always produced in the rear focal plane of the objective lens. The resulting diffraction pattern permits a number of valuable statements to be made about the structure and orientation of the specimen details.
In the case of electron emission microscopy, on the other hand, the specimen is bombarded from the front by primary electrons from the electron source, or by other exciting particles. Therefore, because of the interactions of the primary electrons with the solid being studied, the results are in addition to elastic scattering of the primary electrons, (also secondary electrons and Auger electrons). The released secondary electrons originate primarily from a thin surface layer of the specimen. The backscattered electrons come from the deeper layers of the specimen.
To sample the primary electrons in electron emission microscopy, two different methods were used in the past, and are described in "Historical Perspective and Current Trends in Emission Microscopy, Mirror Microscopy and Low Energy Electron Microscopy" by O. H. Griffith and W. Engel in Ultramicroscopy 36 (1991), pp. 1-28.
One of these methods provides a linear electron-optical system, in which the electron source is mounted behind the detection screen and the primary beam is focused through an aperture in the detection screen onto the specimen. Thus, an electron optical lens is mounted in front of the specimen, in order to align the primary beam in parallel with the optical axis and thus produce parallel illumination of the specimen. In this respect, it is possible to analyze the diffraction pattern (low energy electron diffraction or LEED mode), but the specimen surface cannot be imaged with the elastic electrons (imaging mode or low energy electron reflection microscopy (LEEM) mode), because the reflected ray goes through the aperture in the center of the viewing screen. However, with this microscope, direct imaging of the surface potential with mirror electron microscopy (MEM) is possible. This known linear arrangement is simple in its construction, but the incident beam and the reflected beam cannot be suitably optimized at the same time.
To remedy this problem, a segmented magnetic field has been used in order to be able to separate the incident light and the beam reflected from the specimen. Such an LEEM type electron microscope has been developed by Bauer and Telieps; and an example thereof is described in the above-cited paper in Ultramicroscopy, 36 (1991), p. 22. A further improvement of the LEEM electron microscope for analyzing magnetic surface structures would require an electron source which emits spin-polarized electrons, which are separated in a segmented magnetic field without changing the spin-polarization direction of the imaged electrons. Since spin polarization is necessary, one also refers to a SPLEEM or SPLEED microscope (spin polarized low energy electron microscopy), whose drawback lies, however, in the fact that it is extremely complicated and, therefore, expensive.
Whereas both in the LEED or LEEM, and also in the SPLEEM or SPLEED mode, one works with a parallel electron beam striking the specimen surface, it is necessary for imaging secondary and Auger electrons to focus the primary beam on the specimen, in order to raise the electron density in the visual field. Even with this type of electron microscope, however, the primary beam and the beam of the emitted electrons have to be separated by means of a segmented magnetic field.